Large-scale construction projects involving concrete, such as roadways and dams for example, are now routinely facilitated using fully mobile concrete batching plants. Typically, a mobile batching plant includes aggregate bins, a cement silo and the required weighing and conveying apparatus all mounted to a truss configured as a semi-trailer for portability. Such mobile batching plants are described in my prior U.S. Pat. Nos. 3,872,980 and 3,251,484, the disclosures of which are incorporated by reference herein as if fully set forth.
Portable batching plants can be configured to measure and discharge the concrete ingredients into the drum of a conventional concrete transit mixer truck, with mixing being accomplished on the truck en route to the construction site. Alternatively, the batching plant can be set up s a central mix plant where a mixer at the plant receives the materials, mixes the concrete, and dumps it into conventional dump trucks for transit to the construction site.
The central mix type of operation has heretofore typically included an on-site tilting mixer. This mixer, while adequate in the general sense, suffers from a specific drawback in that a relatively long period of time is required to fill, mix and discharge this kind of mixer. Efficiency in other aspects of large-scale construction have improved to the point where the mixing operation at a central mix plant is the "bottle neck" which establishes the speed of construction. In other words, the raw material supply function and the concrete delivery function are now so efficient that mix time at the central mix plant determines the rate in cubic yards of concrete per day which can be delivered and utilized at the construction site. It is not uncommon to see a central mix plant with a line of dump trucks waiting to receive concrete at a typical central mix batching plant. Thus, there presently exists a need for a concrete mixing apparatus and method which will substantially decrease the amount of time required to load, mix and discharge batches of concrete at a central mix concrete plant.
In addition to the tilt type of mixer now in use, there are numerous other configurations of mixing apparatus that have been used in concrete mixing and/or other environments. For example, Besser Co. of Alpena, Mich. offers a central mix concrete mixer having horizontal spiral ribbons rotating in a horizontally cylindrical drum. The drum is stationary, with a discharge opening in the bottom of the drum opened and closed by a hydraulically operated trap door. The Besser prior art mixers have a typical cycle time of five minutes between batches. U.S. Pat. No. 3,812,821 to Laycock issued May 28, 1974 discloses a tilt type mixing apparatus for producing coated roadstone. U.S. Pat. No. 4,416,545 to Krimmel issued Nov. 22, 1983 discloses a mixer wherein hydraulic motors rotate spiral mixing blades within a horizontal drum and a clutch is used to couple the mixing blade shaft to the drum in order to rotate the drum between a mixing and a dumping position. U.S. Pat. No. 2,155,454 to Temple issued Apr. 25, 1939 discloses a dough mixing apparatus having a horizontally rotatable drum rotated between a mixing and a dumping position by means of a worm gear drive. U.S. Pat. No. 2,165,568 to Muench issued July 11, 1939 discloses a mixing device rotatable between a dumping and a mixing position by means of a worm gear mechanism. U.S. Pat. No. 2,929,658 to Killebrew issued Mar. 22, 1960 discloses a dumpable receptacle mounted on a vehicle for rotation about an eccentric axis by means of a hydraulic cylinder in compression. U.S. Pat. No. 3,100,064 to Kacena issued Aug. 6, 1963 shows a morter-plaster mixer having a mixing container rotatable to a dump position about an eccentric axis by way of a hydraulic cylinder in compression. U.S. Pat. No. 4,331,069 to Tomatis issued May 25, 1982 discloses a curd making machine wherein an eccentrically mounted mixing container rotates to a dump position by way of hydraulic cylinders in compression. U.S. Pat. No. 4,452,536 to Hinkle issued June 5, 1984 discloses a cart for relatively small concrete batches having a container pivotable about an eccentric axis to a dump position by way of a hydraulic cylinder in compression. West German Patent No. 976,415 discloses a mixer having a container which ca be pivoted for discharge about an eccentric axis.